1. Field of the Invention
The present invention relates to an ink set suitable for recording an image by jetting an ink according to an inkjet method, and an image forming method using the same.
2. Description of the Related Art
Various methods have recently been proposed as image recording methods for recording color images. In each method, desired quality of prints is high, including image quality, texture, and post-recording curl.
For example, inkjet techniques have been applied to office printers, home printers, and the like, and have recently been begun to be applied to commercial printing. In commercial printing, printed sheets are required to have a texture similar to that of general printing paper, rather than a surface, such as that of a photograph, that completely blocks penetration of ink solvent into base paper. However, the range of properties such as surface gloss, texture and stiffness is limited when a recording medium has a solvent absorption layer with a thickness as large as from 20 μm to 30 μm. Therefore, application of inkjet techniques in commercial printing has been limited, for example, to posters and vouchers, with respect to which the restrictions on surface gloss, texture, stiffness and the like are tolerable.
Recording media exclusively for inkjet recording have high production costs due to incorporation of solvent-absorbing layers and water-proof layers, which is one of the factors that limit application of inkjet techniques to commercial printing.
A pigment is widely used as a colorant that is one of the components contained in an ink material. When used, the pigment is dispersed in a medium such as water. When the pigment is dispersed and used, the diameter, post-dispersing stability, and size uniformity of the dispersed particles, jettability from jetting heads, and the like are important. Techniques that improve such properties are widely being studied.
There are cases in which pigment-containing inks do not have satisfactory properties in terms of fixability (for example, resistance to rubbing), water resistance, and stain resistance since such inks generally remain on a surface of a recording medium rather than penetrating into the recording medium.
With a view to providing a pigment-containing ink with excellent water resistance and stain resistance, an inkjet ink composition has been disclosed which contains self-dispersing polymer particles (see, for example, Japanese Patent No. 3,069,543).
As a method for providing a pigment-containing ink with excellent resistance to rubbing and water, a method in which a solution containing a polar polymer is applied, together with the pigment ink, onto a recording medium has been disclosed (see, for example, Japanese Patent No. 3,217,486). Another method has also been disclosed in, for example, Japanese Patent No. 3,206,797; in this method, a reaction liquid that reacts with an ink composition, such as a liquid containing a polyvalent metal salt or a polyallylamine, and an ink containing a pigment and a thermoplastic resin emulsion are used to form an image, which is then fixed at a temperature that is not lower than the softening temperature of the thermoplastic resin.
However, satisfactory properties may not be obtained in the method of including self-dispersing polymer particles, the method of applying a solution containing a polar polymer onto a recording medium, and the method of fixing a thermoplastic latex including ink at a temperature not lower than the softening point of the thermoplastic resin. This is because, although the contained polymer or resin works as a binder and improves fixability, water resistance, and the like, the fixability is influenced by thermoplastic properties of the polymer. In particular, when inkjet recording is performed at high speed, the tendency to unsatisfactory properties is conspicuous since sufficient thermal fixing may not be achieved. In such a case, fixing properties can be improved by lowering the softening temperature of the polymer; however, the lowered softening temperature also results in adhesion phenomenon (blocking) of an image surface in a high-temperature environment, and it is difficult to achieve both of improvement in fixability and suppression of the blocking.
The degradation of fixability of the aqueous pigment ink is also affected by the properties of printed sheets. For example, tendency for a pigment to remain on the paper surface is greater and high-speed recording of high-quality images is not possible when the paper is common printing paper used for usual offset printing, which have liquid infiltration property inferior to that of plain paper.
In this relation, a technique of using an ink in which a polymerizable monomer and a pigment are used in combination and curing the ink with UV rays (UV inkjet) is widely known and commonly used. The UV inkjet achieves excellent resistance to rubbing. However, the ink used for the UV inkjet is generally a solvent-based ink, which is environmentally unfavorable, and, when pre-curing ink droplets contact each other, the ink droplets coalesce (spotting interference). Therefore, the UV inkjet is unsuitable for high-speed recording.
Regarding aqueous pigment inks, which are environmentally favorable, a method of improving rubbing resistance by including a polymerizable monomer in an aqueous pigment ink and curing the same is being studied. For example, an inkjet recording method has been disclosed (see, for example, Japanese Patent No. 3,642,152) in which printing is performed on a recording medium by depositing (i) a reaction liquid containing a photopolymerization initiator and a reactive agent that cause coagulation when contacting with an ink composition and (ii) an ink composition containing an acrylate monomer and/or a resin emulsion. Further, an ink of one-component type used for inkjet recording has been disclosed (see, for example, Japanese Patent No. 3,576,862) which includes a specific cationic oligomer and a photopolymerization initiator, together with a colorant and water.
However, according to the method disclosed in Japanese Patent No. 3,642,152, the ink is an aqueous pigment ink in which an aqueous medium is used for the ink, but the solubility of a monomer is not taken into consideration and problems that are caused when a water-soluble monomer is added have not been avoided. Such an ink composition has problems in that the monomer and the pigment are separated from each other, and resultantly a sufficient rubbing resistance of the image is difficult to be achieved, and further blocking properties of the image are poor, and when time has passed in a state where image surfaces are in contact with each other, the images are locally stuck to each other and do not separate. The method includes a reaction between a reaction liquid and an ink to fix the ink. However, since the monomer tends to separate, an immobilization reaction is also insufficient.
In the method disclosed in Japanese Patent No. 3,576,862, the ink includes a specific polymerizable monomer (oligomer) having sufficient water-solubility, thus achieving a condition in which the pigment and the monomer does not readily separate and the rubbing resistance can be readily improved. However, components in the ink before curing have a tendency to coalesce, and the ink is unsuitable for high-speed recording. Regarding this issue, when immobilization of pre-curing ink is attempted using a two-component reaction system as disclosed in Japanese Patent No. 3,642,152, it is difficult to perform sufficient immobilization reaction due to high solubility of the monomer.
When ink droplets are spotted on permeable paper using this method, the pigment remains on the paper surface while the monomer penetrates into the paper, thereby resulting in insufficient rubbing resistance in some cases.